If you are a pfd user you already got denim
No, this isn't correct. You'll have the Operating System component, but the firmware is Denim.
DfP = Operating System
Denim = Firmware
For normal users, both will be installed at the same time. DfP users get one before the other.
I don't believe that Denim is available through the PFD app. If you uncheck your PFD and check, I think you will find you still have Cyan. It's a well known issue.
Denim may or may not carry OS update but the true reason for Denim is firmware update. If you don't have one of the fully supported Denim phones, you will still get the firmware update but just not all the benefits that the 1520, 930 and ICON gets.
It was a well-known bug feel free to search in WindowsCentralMy 925 updated to Denim yesterday after I turned PFD off. The update didn't ask for consent as it normally does, but when I updated the extras& info and checked it today morning, it showed Lumia Denim.
I think you're confusing the issue here.
Customers will still get the Denim firmware, regardless of whether there is hardware to support the major Denim features.